1. Field of the Invention
The invention relates generally to plasma spraying. In particular, the invention relates to joining silicon parts used in semiconductor fabrication equipment.
2. Background Art
Batch substrate processing continues to be used in fabricating semiconductor integrated circuits and similar micro structural arrays. In batch processing, many silicon wafers or other types of substrates are placed together on a wafer support fixture in a processing chamber and simultaneously processed. Currently most batch processing includes extended exposure to high temperature, for example, in depositing planar layers of oxide or nitride or annealing previously deposited layers or dopants implanted into existing layers. Although horizontally arranged wafer boats were originally used, vertically arranged wafer towers are now mostly used as the support fixture to support many wafers one above the other.
In the past, the towers and boats have been most often made of quartz or sometimes of silicon carbide for high-temperature applications. However, quartz and silicon carbide have proven unsatisfactory for many advanced processes. An acceptable yield of advanced integrated circuits depends upon a very low level of particles and metallic contaminants in the processing environment. Often the quartz towers develop excessive particles after a few cycles and must be reconditioned or discarded. Furthermore, many processes require high-temperature processing at above 1000° C. or even above 1250° C. Quartz sags at these high temperatures although silicon carbide maintains its strength to a much higher temperature. However, for both materials the high temperature activates the diffusion of impurities from the quartz or silicon carbide into the semiconductor silicon. Some of the problems with silicon carbide have been solved by coating the sintered SiC with a thin SiC surface coating deposited by chemical vapor deposition (CVD), which seals the contaminants in the underlying sintered silicon carbide. This approach, despite its expense, has its own problems. Integrated circuits having features sizes of 0.13 μm and below often fail because slip defects develop in the silicon wafer. It is believed that slip develops during initial thermal processing when the silicon wafers are supported on towers of a material having a different thermal expansion than silicon.
Many of these problems have been solved by the use of silicon towers, particularly those made of virgin polysilicon, as described by Boyle et al. in U.S. Pat. No. 6,450,346, incorporated herein by reference in its entirety. A silicon tower 10, illustrated orthographically in FIG. 1, includes three or more silicon legs 12 joined at their ends to two silicon bases 14. Each leg 12 is cut with slots to form inwardly projecting teeth 16 which slope upwards by a few degrees and have horizontal support surfaces 18 formed near their inner tips 20. A plurality of wafers 22, only one of which is illustrated, are supported on the support surfaces 18 in parallel horizontal orientation along the axis of the tower 10. For very high-temperature processing, it is preferred that there be four legs 12 and that the support surfaces 18 be arranged in a square pattern at 0.707 of the wafer radius from the center. A boat has much the same structure but with both bases configured on one side to support the horizontally arranged boat. The wafers are supported a few degrees from vertical both at the bottom of the slots and the tips of the teeth.
Superior results are obtained if the legs 12 are machined from virgin polysilicon (virgin poly), which is bulk silicon formed by chemical vapor deposition with silane (SiH4) or a chlorosilane (SiClH3, SiCl2H2, SiCl3H, or SiCl4) as the precursor. Virgin poly is the precursor material formed in multi-centimeter ingots, which is used for the Czochralski growth of silicon ingots from which wafers are cut. It has an exceedingly low level of impurities. Although virgin poly would be the preferred material for the bases 14, it is not usually available in such large sizes. Czochralski silicon may be used for the bases 14. Its higher impurity level is of lesser importance since the bases 14 do not contact the wafers 22.
Fabricating a silicon tower or boat, particularly out of virgin poly, requires several separate steps, one of which is joining the machined legs 12 to the bases 14. As schematically illustrated in FIG. 2, blind mortise holes 24 are machined into each base 14 with non-circular shapes in correspondence with and only slightly larger than ends 26 of the legs 12. Boyle et al. favor the use of a spin-on glass (SOG) that has been thinned with an alcohol or the like. The SOG is applied to one or both of the members in the area to the joined. The members are assembled and then annealed at 600° C. or above to vitrify the SOG in the seam between the members.
SOG is widely used in the semiconductor industry for forming thin inter-layer dielectric layers so that it is relatively inexpensive and of fairly high purity. SOG is a generic term for chemicals widely used in semiconductor fabrication to form silicate glass layers on integrated circuits. Commercial suppliers include Allied Signal, Filmtronics of Butler, Pa., and Dow Corning. SOG precursors include one or more chemicals containing both silicon and oxygen as well as hydrogen and possibly other constituents. An example of such as precursor is tetraethylorthosilicate (TEOS) or its modifications or an organo-silane such as siloxane or silsesquioxane. In this use, it is preferred that the SOG not contain boron or phosphorous, as is sometimes done for integrated circuits. The silicon and oxygen containing chemical is dissolved in an evaporable carrier, such as an alcohol, methyl isobutyl ketone, or a volatile methyl siloxane blend. The SOG precursor acts as a silica bridging agent in that the precursor chemically reacts, particularly at elevated temperature, to form a silica network having the approximate composition of SiO2.
Boyle has disclosed an improvement of the SOG joining method in U.S. provisional application, Ser. No. 60/465,021, filed Apr. 23, 2003 and incorporated herein by reference in its entirety. In this method silicon powder is added to the liquid SOG precursor to form a slurry. Terpineol alcohol is added to slow the setting time. The powder preferably has a particle size of between 1 and 50 μm and is prepared from virgin polysilicon. The slurry adhesive is applied to the joint before assembly and is cured similarly to the pure SOG adhesive to form a silica/polysilicon matrix with the polysilicon fraction being typically 85% or greater. The improved SOG/polysilicon adhesive is believed to be stronger than the pure SOG adhesive and contains a significantly lower fraction of silica originating from the SOG, thereby reducing the contamination problem. Nonetheless, a certain amount of silica remains, thereby reducing but not eliminating contamination and the tendency of the joint to dissolve in HF.
Two silicon members to be joined are separated by a gap having a thickness of about 50 μm (2 mils). The thickness of the gap represents an average separation of the leg 12 and the base 14 as the end 26 of the leg 12 is at least slidably fit in the mortise hole 24. The gap thickness cannot easily be further reduced because of the machining required to form the complex shapes and because some looseness of assembled members is needed to allow precise alignment of the support surfaces and other parts. A coating of the liquid SOG precursor or the SOG/silicon-powder mixture is applied to at least one of the mating surfaces before the two members 12, 14 are assembled such that the SOG precursor with optional silicon powder fills the gap 34 of FIG. 3. Following curing and a vitrification anneal at a temperature typically above 600° C., the SOG precursor with optional silicon powder changes into a solid having the structure of a silicate glass in a three-dimensional network of silicon and oxygen atoms and their bonds and optionally forming a matrix for the larger fraction of the embedded silicon crystallites.
Silicon towers and boats produced by this method have provided superior performance in several applications. Nonetheless, it is possible that the bonded structure and in particular the bonding material may still be contaminated. The very high temperatures experienced in the use or cleaning of the silicon towers, sometimes above 1300° C., may worsen the contamination. One possible source of the contaminants is the relatively large amount of SOG used to fill the joint between the members to be joined. Siloxane SOG typically used in semiconductor fabrication is cured at around 400° C. and the resultant glass is not usually exposed to high-temperature chlorine. However, it is possible, though the effect has not been verified, that the very high temperature draws out the few but possibly still significant number of contaminants in the SOG. The SOG/silicon mixture reduces the amount of SOG but does not eliminate it.
Some integrated circuit fabrication facilities require periodic cleaning of towers in hydrofluoric acid (HF). Silica, however, tends to be etched by HF so that SOG-bonded towers may come apart after HF cleaning.
Silicon towers need to be assembled with alignment tolerances of typically of the order of 25 μm in order to support wafers without rocking. Large mechanical jigs are used to align the members of an assembled towers before the bonding between the members is completed. A SOG adhesive presents two fabricational difficulties in maintaining the alignment. Typically the spin-on partially hardens or cures at room temperature in less than an hour. The hardening time can be lengthened somewhat by diluting the commercially available SOG precursor with alcohol or the like. Nonetheless, only about an hour is available to apply the SOG to the joining members, to assemble the members, and to align the members in the jig. While such quick fabrication is possible, it leaves little room for error or unexpected delays and impacts work scheduling. Furthermore, the alignment should be maintained during the final curing of the spin-on glass at 600° C. and typically even higher at 1200° C. As a result, the alignment jig should support the tower in the annealing furnace. Therefore, either the alignment is performed in a cooled furnace, which is thereafter raised to the curing temperature, or the jig and its supported assembled tower is inserted into a furnace, which may be kept at a somewhat elevated temperature. Again, placing the jig with its supported tower into an annealing furnace is possible, but such a process is inconvenient and slows throughput.
In U.S. Pat. No. 6,284,997, Zehavi et al. have disclosed a method of welding together silicon members, thereby avoiding the use of SOG and or a SOG/silicon mixture and their potential drawbacks. However, Zehavi et al. teach that cracks can be avoiding in welding silicon only by pre-heating the silicon members to at least 600° C. before the welding step heats the localized area of the weld seam to above the melting point of silicon, 1416° C. The welding method has proven successful at producing crack-free welds essentially free of contamination. However, welding 600° C. members is a difficult and unpleasant process. Furthermore, the 600° C. pre-heating needs to be performed with the members held in the alignment jig. So again, silicon welding is possible but has its drawbacks.
Siemens et al. in U.S. Pat. No. 5,070,228 disclose the use of plasma spraying to join parts composed of a limited number of specified reactive metals. The method has limited applicability to complex structures and requires pre-heating the parts in a non-reactive environment using a complex apparatus.